Radicals from Electron Impact on Fluorocarbons

Abstract

In a low-pressure, nonequilibrium plasma in a fluorocarbon gas such as that used in a processing reactor in semiconductor device fabrication, the active chemical species are all fragments of the feed gas. the fluorocarbon feed gas is itself chemically benign and the parent ions are unstable and do not persist for a sufficient time to contribute to plasma chemistry. the active fragments are produced by electron impact in the 5 to 30 eV energy range typical of the electron-energy distribution in the plasma. in general, electron-molecule collisions leading to dissociation include dissociative ionization, neutral dissociation, dissociative electron attachment, and dipolar dissociation. for fluorocarbons, only the first two are important. Dissociative attachment must be initiated by the resonant attachment of an electron to yield a negative ion that is sufficiently long-lived for dissociation to compete with autodetachment of the electron. Resonant electron attachment is not a significant feature of electron-fluorocarbon interactions. Dipolar dissociation proceeds through very high-lying electronic states. the cross sections are generally small, especially for electrons with energies less than about 30 eV. Although not a mathematical certainty, it is generally observed that the neutral fragments from both dissociative ionization and neutral dissociation are odd-electron species; they are radicals. Chemically they are very reactive. in addition, in comparison to ions, these species have relatively low sticking probabilities at surfaces. As a consequence radicals may persist in the plasma for a much longer time than ions. the concentration of radicals in a plasma can exceed the concentration of ions by as much as four orders of magnitude.